If it’s for driving thrills/pleasure £10k – £15k on a monster Westfield or Caterham would be the way to go. Spend the change on track days and spares, decent tyres and pads are seriously expensive. Or if you’re looking for a big torque-pedo then AMG mercs are stupidly cheap. You can easily get 500bhp, low mileage cars for your budget. But maintenance.
If it’s got to be Porsche I’d go Cayman. Porsche seem to be doing a good job of fostering their reputation as a drivers car with the GTS and GT4 so that should help keep their values up, whereas the Boxster always has that trophy wife/hairdresser vibe. Either way you’d need to know your onions spending £20k on a car that cost £70k.
I’d be avoiding luxury marques and looking at something that will hold it’s value or appreciate, and would have said NSX/Escort Cosworth etc a couple of years ago, but now they’ve gone through the roof. Maybe a hot MK1 or MK2 escort before they all get to silly money?